Control of a passive prosthetic knee joint with adjustable damping

ABSTRACT

The invention relates to the control of a passive prosthetic knee joint with adjustable damping in the direction of flexion such that a prosthetic unit attached to the knee joint can be adapted for climbing stairs.

CROSS-REFERENCE TO RELATED APPLICATION

This patent application is a national stage application of InternationalApplication PCT/DE2007/000841, filed on May 8, 2007, which claimspriority to German Patent Application No. 10 2006 021802.7, filed on May9, 2006. The entire contents of both applications are hereby expresslyincorporated by reference.

TECHNICAL FIELD

The invention relates to the control of a passive prosthetic knee jointwith adjustable damping in the direction of flexion such that aprosthetic unit, with upper attachment elements and with a connectionelement to an artificial foot, which elements are secured on theprosthetic knee joint, can be adapted for climbing stairs.

BACKGROUND

Prosthesis wearers who require a knee prosthesis have to acceptcompromises in terms of the damping of the flexion and extension in theconfiguration of their prosthesis, since passive prosthetic knee jointsare optimized only for certain uses, and, therefore, significantlydifferent movement patterns are either not possible or are only possiblewith extraordinary difficulty. Thus, the movement sequence for walkingon level ground, for which most of the passive prosthetic knee jointswith flexion and extension damping are designed, requires substantiallydifferent damping characteristics than those for climbing stairs.Therefore, with the conventional knee joint prostheses, the prosthesiswearer climbs stairs by a procedure in which, standing in front of thestairs, the healthy leg is lifted onto the first step and thecontralateral leg is then pulled up onto this same step. The walkingspeed may possibly be increased if the healthy leg is placed on everysecond step, but this is very demanding.

In conventional knee joint prostheses designed for walking on levelground, the necessary low extension damping of the prosthetic knee jointhas the effect that, when climbing stairs, an abrupt extension takesplace when pushing oneself upward, and this subjects the prosthesiswearer to an unacceptably high load. Balancing of the leg provided withthe prosthesis is also not possible, because three joints, namely theankle joint, the knee joint and the hip, are arranged one above theother and the prosthesis wearer can only directly control the hip joint.Even when lifting the prosthetic foot in order to reach the next stepup, the problem arises that the prosthetic foot is moved onto the riseror onto the underside of the next step tip, since the necessary flexiondamping in the swing phase control for walking on level ground makes itimpossible to reach the top face of the next step up. The flexion drivemechanism provided in active knee joints for the purpose of lifting thefoot and the extension drive mechanism for straightening the knee andlifting the body via the leg provided with the prosthesis is verycomplicated and very heavy. Furthermore, the swing phase control forwalking on level ground is very limited in these active prosthetic kneejoints.

SUMMARY

An object of the present invention is to make available a control modefor a passive knee joint with which an alternating climbing of stairs ispossible for a prosthesis wearer. According to the invention, thisobject is achieved by a control having the features of claim 1.Advantageous embodiments and developments of the invention are set forthin the dependent claims.

In the control mode according to one embodiment of the invention, apassive prosthetic knee joint with adjustable damping in the directionof flexion allows a prosthetic unit, with an upper element attachable tothe knee joint and a connection element to an artificial foot, to beadapted for climbing stairs. First, a low-torque lift of the prostheticfoot is detected. After the detection of a low-torque lift of theprosthetic foot, flexion damping in the lift phase is lowered,specifically to a level below that which is suitable or optimized forwalking on level ground. By lowering the flexion resistance duringlifting of the prosthetic foot, it is possible to obtain a knee anglethat allows the prosthetic foot to be more easily placed on the nextstep up. More particularly, a flexion of the hip, the low-torque lift ofthe prosthetic foot, and the mass inertia of the prosthetic foot,results in a passive prosthetic knee joint angle which, by bringingforward the hip or by a corresponding extension through the force ofgravity, is sufficient to negotiate the step edge and to position theprosthetic foot over the step. It is advantageous in this case for theweight distribution in the prosthesis to be configured such that thecenter of gravity is arranged as far as possible in the distaldirection, for example in the connection element to the prosthetic footor in the prosthetic foot itself. To this end, for example, the controlunit of the knee system can be arranged distally instead of near theknee such that, without increasing the weight of the prosthesis throughextra weights in the prosthetic foot, the desired effect of the kneeflexion is achieved with a low-torque lift of the prosthetic foot.

During a subsequent foot placement and hip-straightening phase, theflexion damping and optionally the flexion extension is increased toallow the prosthesis to be straightened. In one embodiment, the flexiondamping and possibly the extension damping is increased to a level abovea damping for a swing phase control for walking on level ground, suchthat a controlled extension or straightening of the hip joint, kneejoint and of the ankle joint can take place.

After the step edge has been negotiated, the knee is straightenedthrough the force of gravity. To permit positioning of the prostheticfoot located over the step, the flexion damping is increased prior tostraightening the prosthetic knee joint, such that the prosthetic footcan be positioned via the hip angle directly controlled by the patient.

In the foot placement phase and, if appropriate, the hip-straighteningphase, the flexion damping is preferably increased to a maximum value toreduce or avoid a lowering caused by an insufficient hip-straighteningtorque. In one embodiment, the flexion damping in the foot placement andhip-straightening phase is maintained until the hip is completelystraight.

In one embodiment, extension damping is set during the lift phase, footplacement phase and hip-straightening phase. During the lift phase,extension damping is set to avoid a gravity-induced straightening of theprosthetic knee joint or a dropping of the prosthetic foot. During thefoot placement and hip-straightening phase, extension damping is set toposition the foot down in a controlled manner. In contrast, if extensiondamping was completely absent during the hip-straightening phase, thiswould result in an unnatural upward bounding movement of the patient,which would lead to a stop and an abrupt interruption in maximumextension of the knee joint. In open steps, that is to say withoutrisers, a lifting of the prosthetic foot without extension damping canhave the effect that the prosthetic foot is pushed under the next stepup.

The flexion damping is preferably increased as a function of the changeof the knee angle. As soon as a defined knee angle is reached, which isgenerally greater than a knee angle suitable for walking on level groundin a swing phase control, the flexion damping is increased.Alternatively or in addition, the flexion damping can be increased orlowered as a function of the axial force acting on the lower leg shaft.If the axial force drops sufficiently quickly to approximately zero withthe knee almost straightened, this is an indicator for initiation of astair-climbing mode.

In addition or alternatively to this, the vertical acceleration of theleg, that is to say the thigh or lower leg and hip, can take place witha simultaneous drop in axial force from triggering for activation of acorresponding flexion damping control and extension damping control forclimbing stairs. Moreover, a sufficiently rapid bending of the hip cancause the knee to bend with little or no axial force. Instead of anaxial force, knee-straightening torque, ankle torque or a combination ofthe forces and torques can be detected to initiate the stair-climbingmode.

The detection of a low-torque lift can be achieved mechanically via acaliper or via a force or torque sensor. The caliper can be designed,for example, as a slide which is mounted in a guide and which, withalmost perpendicular lifting of the prosthetic foot, moves into aswitching position that reduces the flexion damping. The measurement ofthe forces or torques can be achieved by known sensor devices.Alternatively, the low-torque lift can take place via a measurement ofthe horizontal acceleration of the prosthetic foot and the detection ofa bending in the knee joint. In contrast to walking on level ground, alow horizontal acceleration of the prosthetic foot, that is to say withalmost perpendicular lifting, causes a high bending to take place in theprosthetic knee joint, which indicates stair-climbing. Moreover, thetorque at the front of the prosthetic foot can be detected to determinewhether the prosthesis user would like to move in a horizontal directionduring walking resulting in a very high loading of the front of theprosthetic foot, or whether there is a reduced axial force and a flexionin the knee joint with the prosthetic foot set down.

To achieve the necessary flexion for negotiating the step height afterthe lift, a flexion support in the lift phase can be achieved via apretensioned spring or another force-storing mechanism. Likewise, freeextension can be supported by a spring when a certain time has elapsedafter lowering the flexion damping. This is necessary for safetyreasons, to prevent unwanted damping control in the event of erroneoustriggering of the stair-climbing mode.

The increase of the flexion damping and if appropriate of the extensiondamping is initiated when the prosthetic foot, after being lifted, isplaced down again, for example if an increase of the axial force isdetermined. Alternatively, with the knee angle remaining more or lessconstant, the extension and flexion damping can be increased.

The flexion damping in the lift phase can be lowered to a minimum value,such that the damping effective in each system is not further increasedon account of friction.

The detection both of the low-torque or torque-free lift and also of thelowering of the flexion damping can be done mechanically, and similarlythe changing of the various damping, in order to permit a prosthesisconstruction that is as simple as possible.

BRIEF DESCRIPTION OF THE DRAWINGS

An illustrative embodiment is explained in more detail below withreference to the figures.

FIGS. 1 to 6 are schematic depictions showing the sequence involved inalternating stair-climbing with a passive knee joint prosthesis.

DETAILED DESCRIPTION

FIG. 1 shows a prosthesis wearer 1 with a knee joint prosthesis 2 whichis secured by upper attachment elements to a femoral stump. Theprosthetic leg 20 stands with the healthy contralateral leg 4 in frontof a step.

To reach the next step up, a prosthetic foot 6 has to be guided over thestep edge. An active bending of the hip, as is indicated by the arrow 7,assists the passive bending of the knee, which is shown by the arrow 8and which, because of the mass inertia both of the prosthetic foot 6 andalso of the connection element 3, occurs from the prosthetic knee joint2 to the prosthetic foot 6. For this purpose, a minimum extensiondamping is required to ensure that, after a flexion of the hip, theprosthetic foot 6 does not swing forward and is not moved against theriser or under the step 5. In the lift phase, as shown in FIG. 2, theprosthetic foot 6 is guided upward, as far as possible in aperpendicular manner, this possibly being initiated by a slight rearwardmovement. The lift is detected via the flexion angle α between theconnection element 3 and the thigh or via a reduction of the axial forcein the connection element 3, without flexion of the prosthetic foot 6.It is also possible to detect the stair-climbing mode, and thus thelowering of the flexion damping to a value below the normal swing phasecontrol, preferably to the minimum value, via a horizontal rearwardmovement of the prosthetic foot 6 in conjunction with a bending of thehip.

After the step edge has been negotiated and the lift phase completed, asis shown in FIG. 2, a secure positioning of the prosthetic foot 6 on thestep is required. For this purpose, the prosthetic foot 6 has to bemoved forward, which can be achieved by extension as a result of theforce of gravity. For this purpose, an extension damping can be reduced,if this has not already been done in the lift phase. A prosthetic kneejoint 2 that is sufficiently damped in flexion and extension prior tostraightening allows the prosthesis wearer 1 to position the prostheticfoot 6, by changing the hip angle. In the lowering and hip-straighteningphase, the flexion and extension are preferably strongly damped tocontrol the foot placement, and to prevent a spontaneous backward fallin the event that the hip-straightening torque is insufficient. Theextension remains damped so as to be able to control the speed ofstraightening of the hip and knee. This is shown in FIG. 3.

In FIG. 4, the foot placement phase is completed. The prosthesis wearer1 can initiate straightening of the knee with a hip-straighteningtorque. The straightening of the knee can be assisted by an extension ofthe healthy foot.

FIG. 5 shows the increasing straightening of the knee throughapplication of a hip torque. The increasing straightening of the kneeshortens the effective lever and facilitates the straightening of theknee through the straightening of the hip.

FIG. 6 shows the complete extension of the leg provided with the kneejoint prosthesis 2. The contralateral leg 4 is moved past the prostheticleg 20 and placed on the next step up, such that alternating climbing ofstairs is possible with the passive knee joint prosthesis.

Accordingly, the control is configured in such a way that, during thelift of the prosthetic foot 6 a flexion resistance is set that resultsin a knee angle α, which allows the prosthetic foot 6 to be placed onthe next step. Flexion support by spring mechanisms may facilitate thelifting movement and make it easier to negotiate the step height.

If no action is to take place after the stair-climbing mode has beentriggered by detection of a low-torque lift, a free extension is set,said free extension being set in a time-dependent manner. The timefunction can also be mechanical. The low-torque lift can be detected viathe mass inertia, if the healthy leg is first set down and only thesecond step is intended to be negotiated by the leg provided with theprosthesis. If the prosthetic foot is first unloaded and the prostheticknee joint then bent, the stair-climbing mode is set. Damping both inthe direction of extension and also in the direction of flexion afterthe lift phase, that is to say during the hip-straightening phase, ismaintained until a complete extension of the prosthetic knee joint isreached or detected.

1. A method for initiating and implementing a stair-climbing mode in apassive prosthetic knee joint connected to a prosthetic leg unitincluding a prosthetic foot comprising: detecting a low-torque lift ofthe prosthetic foot; initiating a lift phase, in which a flexion dampinglevel of the knee joint is reduced to a level below that which is usedfor walking on level ground; detecting a placement of the prostheticfoot; and initiating a lowering phase, in which the flexion dampinglevel is increased to a level above that which is used for walking onlevel ground.
 2. The method of claim 1, wherein in the lowering phase,an extension damping level is increased to a level above that which isused for walking on level ground.
 3. The method of claim 2, wherein theflexion damping are increased to maximum levels in the lowering phase.4. The method of claim 1, wherein the flexion damping is reduced to aminimum level in the lift phase.
 5. The method of claim 1 furthercomprising the step of, during the lowering phase, maintaining theflexion damping level until a straightened hip is detected.
 6. Themethod of claim 1, further comprising the step of, during the loweringphase, detecting a knee angle and establishing the flexion damping levelas a function of the detected knee angle.
 7. The method of claim 1wherein at least one of the detecting steps comprises the step ofdetecting an axial force along the prosthetic unit.
 8. The method ofclaim 1 wherein an extension damping level is established during each ofthe lifting and lowering phases.
 9. The method of claim 1 wherein thestep of detecting the low torque lift comprises measuring a horizontalacceleration of the prosthetic foot and by detecting a bend in the kneejoint.
 10. The method of claim 1 wherein the step of detecting the lowtorque lift comprises detecting a torque at the front of the prostheticfoot.
 11. The method of claim 1 wherein the step of detecting theplacement of the foot comprises measuring an axial force measurementalong the prosthetic unit.
 12. The method of claim 1 wherein afterinitiating the lifting phase, a time controlled free extension is set.